A modern method of forming integrated circuit devices involves the formation of semiconductor devices on a thin film of single crystalline semiconductor material separated from a substrate of semiconductor material by a layer of buried oxide or insulator. This is commonly referred to as semiconductor on insulator construction or SOI construction. SOI construction allows for high performance semiconductor devices due to, among other reasons, the reduction in junction capacitance between diffused areas within the devices and the substrate.
The thickness of the thin film of single crystalline semiconductor material is an important consideration in SOI construction. One method of SOI construction is called bond-and-etch-back construction or BESOI construction. Various approaches to BESOI construction are known. One method begins with two separate pieces of semiconductor material, namely, an outer semiconductor layer and a substrate. An etch stop dopant is implanted into the outer semiconductor layer. A first insulator layer is formed outwardly from the doped region of the outer semiconductor layer. A second insulator layer is formed outwardly from the semiconductor substrate. The outer semiconductor layer is then inverted such that the first insulator layer is disposed inwardly from the outer semiconductor layer. The surfaces of the first and second insulator layers are bonded by placing the surfaces together under appropriate temperature and pressure. The bonded combination of the first and second insulator layers forms a buried insulator layer separating the outer semiconductor layer from the semiconductor substrate. Finally, an exposed surface of the outer semiconductor layer opposite the buried insulator layer is etched back toward the buried insulator layer with an etchant for which the etch rate is a function of the impurity type. A thin film of semiconductor material is left disposed outwardly from the buried insulator layer with a thickness dependent in part on the depth of the original etch stop implant. According to this technique, it is difficult to control the thickness of the semiconductor film. This technique limits the processing that can be done subsequent to the etch stop implant and prior to the etch back to processes that do not cause significant diffusion of the etch stop dopant, for example, low temperature processes. It has been shown that nitridation of the interface between an oxide and a semiconductor layer improves radiation hardness. However, nitridation of the interface between the buried oxide and the outer semiconductor layer is precluded because of the effect it could have on the etch stop dopant distribution. Additionally, it is difficult to implement structures in the buried insulator layer due to alignment difficulties with devices fabricated on the semiconductor film. For example, it is sometimes desirable to make contact to the channel region of an SOI transistor. The efficiency of doing this with a buried conductor depends on the alignment accuracy of the buried conductor contact to the channel region.
Therefore, a need has arisen for a semiconductor on insulator device and a method for forming a semiconductor on insulator device that provide accurately controlled thickness of the outer semiconductor layer, allow processing of the outer semiconductor layer prior to bonding, and provide implementation of buried interconnects.